1. Field of the Invention
The present invention relates to a photosensitive resin composition, a flexible circuit board employing the photosensitive resin composition, and a circuit board production method.
2. Description of the Related Art
A flexible circuit board on which an electronic component such as a semiconductor element is mounted by soldering generally includes a surface protective layer (cover insulating layer) provided on a surface thereof formed with an electrically conductive pattern. The surface protective layer is typically formed by stamping a polyimide film formed with an adhesive layer (so-called cover lay film) into a predetermined shape and bonding the film on the surface of the flexible circuit board. Where the cover insulating layer is formed as having a fine opening pattern (conformal to openings provided in the board), the formation of the cover insulating layer is achieved by forming a film of a heat-resistant material called “solder resist” on a necessary portion of the electrically conductive pattern and curing the solder resist film by a screen printing method and an exposure/development method. The cover insulating layer is required to have an insulative property and solder heat resistance during the soldering of the electronic component.
The flexible circuit board is incorporated in an electronic device such as a mobile phone after the mounting of the electronic component. The flexible circuit board is often folded so as to be properly accommodated in a narrow space in the electronic device. In this case, the cover lay film, which is excellent in folding endurance, is selected as a cover insulating layer to be provided in a foldable portion of the board. However, the solder resist film, which is poorer in folding endurance, is rarely selected.
Thus, the cover lay film and the solder resist are selectively used for forming parts of the cover insulating layer of the flexible circuit board according to necessary properties such as the folding endurance and the fineness of the opening pattern. Therefore, it is a common practice to use the cover lay film and the solder resist in combination.
However, the combinational use of the cover lay film and the solder resist increases the number of steps of the production process, making it difficult to reduce production costs.
To solve this problem, the applicant of the present invention previously proposes several photosensitive resin compositions excellent in flexibility and suitable for use as a solder resist (see JP-A-2006-235371 and JP-A-2006-301186).
However, the photosensitive resin compositions disclosed in JP-A-2006-235371 and JP-A-2006-301186 each have a lower pyrolysis temperature and, hence, are unsatisfactory in folding endurance after an IR reflow process (in which a solder is reflowed by far infrared radiation). Therefore, there is still room for improvement in this respect.
In view of the foregoing, it is an object of the present invention to provide a photosensitive resin composition which has properties necessary for the solder resist (insulative property, solder heat resistance, alkali developability and the like) and is capable of forming a film that is excellent in folding endurance even after the IR reflow process, and to provide a flexible circuit board employing the photosensitive resin composition and a circuit board production method.